If you watch any All Stars table one of the first things that you will notice is that it's very rare to see any of the players just calling preflop (also known as 'limping in'). In the vast majority of hands the first player into the pot will open with a raise, usually anywhere between 2-4 big blinds. This is a tactic that you should look to learn from the next time you play in a cash game, for a number of reasons.
If you only raise with your strong hands, and limp in with every mediocre hand that you want to play, then it's going to be very easy for your opponents to read you. Whenever they see you limp they will raise big and get you to fold preflop or have to commit more chips than you intend on doing just to see a flop. Instead, if you raise preflop with all hands that you wish to play, it's going to be impossible to get a read on you. You could have anything from pocket Aces to a suited connector to a small pair – it will be impossible to tell.
More ways to win
Aside from disguising your hand, one of the other key benefits to raising instead of calling is that it gives you the opportunity to win the blinds right away without even seeing a flop. Now, if you have a very strong hand this won't always be the most desirable outcome but if you're raising something marginal such as K-9 (perhaps because it has been folded to you in late position) then picking up the blinds without a battle is not a bad result at all. These small wins may not seem like much but if you are continually picking up blinds it will add up to a tidy sum over time. When you just call the option to win the blinds is taken away from you, and generally you're going to have to see a flop to take down the pot.
When you raise preflop it can have positive knock-on effects for the rest of the hand too. Let's say you raise with 7-6 and get one called. The flop is A-2-4. You've missed so it's time to give up, right? Wrong! By raising preflop you have started to tell a story that you have a decent hand, one that could very credibly contain an Ace. If your opponent checks to you, then this is an excellent opportunity to make a continuation bet and try to steal this pot. If your opponent is sat there with a better hand, such as King-high or even a medium pocket pair, they will often give you credit for hitting the Ace and just fold. Of course, if you are called you should be prepared to give it up on the turn unless your hand improves or you sense weakness.
Compare this to if you had just limped in preflop. Even if the flop is exactly the same and your opponent checks it to you, now any bluff that you make has much less credibility. Surely if you had an Ace you would have raised preflop?
Sometimes it's easy to treat poker hands as a street-by-street puzzle that needs solving, but this is a mistake. Where possible you should attempt to have a plan for the entire hand – a plan that often starts with the aggressive action of raising preflop.